Add documentation for mem_limit

[kadel]

Add documentation for #414

[pradeepto]

Bumped into this issue while I was discussing other issues, I think we can close this issue. What do you think @kadel ?

[cdrage]

@pradeepto Still pending for this being merged #448 as it's part of the documentation :)

[kadel]

#448 was merged. That should be enough to document it for now.